FIG. 1 illustrates a conventional thermoelectric device 20 that includes a thermoelectric couple 24 thermally coupled between a hot-side thermal interface member 28 and a cold-side thermal interface member 32. During use, hot-side thermal interface member 28 thermally communicates with a region or structure, e.g., a heat source 36, having a first temperature, and cold-side thermal interface member 32 thermally communicates with a region or structure, e.g., a heat sink 40, having a second temperature lower than the first temperature. Thermoelectric couple 24 includes a first thermoelement 44 and a second thermoelement 48. Often, but not necessarily, one of thermoelements 44, 48 is made of a p-type semiconductor material and the other is made of an n-type semiconductor material. Accordingly, first and second thermoelements 44, 48 are labeled, respectively, “P” and “N” as a matter of convenience for denoting that the two thermoelements are made of different materials. First and second thermoelements 44, 48 are typically electrically coupled to one another at one end by a tie contact 52 and include contacts 56, 58 at their respective other ends. Contacts 56, 58 are typically also tie contacts that electrically connect each of first and second thermoelements 44, 48 to corresponding adjacent thermoelements (not shown). Typical thermoelectric devices often comprise tens, hundreds or more thermoelectric couples.
Thermoelectric devices, such as thermoelectric device 20 of FIG. 1, operate via two general phenomena: the Seebeck effect and the Peltier effect. The Seebeck effect is where a voltage is observed when two dissimilar materials, e.g., the dissimilar materials (P and N) of first and second thermoelements 44, 48, are coupled electrically in series and thermally in parallel, and are subjected to a thermal gradient across the thermoelements. This voltage is proportional to the temperature gradient across device 20 and a property of each material known as the Seebeck coefficient.
The Peltier effect is a second, complementary effect in which a temperature difference is observed at the interface of two dissimilar thermoelectric materials, e.g., the two dissimilar materials (P and N) of second and first thermoelements 44, 48, when an electric current passes through a circuit comprising the thermoelements. Generally, the Seebeck effect is exploited to generate power from thermal gradients, and the complementary Peltier effect is exploited for cooling or heating applications, given a power source.
As a thermal engine, the performance of a thermoelectric device, such as thermoelectric device 20, is limited by the thermodynamic Carnot efficiency (ηc=[THot−TCold]/THot). Presently available solid-state thermoelectric devices operate at only a fraction of the potential Carnot efficiency, with overall efficiencies ranging from 3-15%. Efficiencies of this order make it exceedingly difficult for thermoelectric devices to compete with traditional methods of heating, refrigeration and power generation. The most significant contributor to such poor efficiencies is the conduction of heat through thermoelements 44, 48 from the hot to the cold side.
There are two modes by which heat conduction through thermoelements 44, 48 causes a reduction in the overall thermodynamic efficiency of thermoelectric device. The first mode concerns the distribution of heat throughout thermoelements 44, 48. In the case of power generation, heat is the energy source that thermoelectric couple 24 converts into electrical energy. Therefore, any heat conducted away from heat source 36 and distributed throughout thermoelements 44, 48 is energy that potentially could have been converted into electrical energy.
The second and more significant mode resulting in reduced efficiency due to heat conduction is the compromise of the temperature differential between hot side member 28 and cold side member 32 of thermoelectric device 20. According to the Seebeck effect, the voltage across a thermoelectric couple is directly proportional to the temperature gradient. Ideally, the temperatures of hot and cold side members 28, 32 would always remain constant, i.e., both temperatures would be unaffected by heat transfer through thermoelements 44, 48. Unfortunately, this is not realistic and heat transfer through thermoelements 44, 48 can greatly affect the temperature of either or both of hot and cold side members 28, 32. Thus, if the temperature of cold side member 32 increases due to heat conduction, the temperature gradient between hot side member 28 and the cold side member will decrease, resulting in a smaller voltage and less power generated. In one aspect, the present invention seeks to significantly increase thermoelectric device efficiencies by greatly reducing heat transfer through the thermoelements.
The present inventors are presently aware of only two known technologies in the context of thermoelectric devices that attempt to insulate against thermal conduction. The first technology utilizes thermionic emission using semiconductor thermal diodes and a complex microstructure. These materials are difficult and expensive to manufacture. There are several patents on this technology, including U.S. Pat. No. 6,396,191 to Hagelstein, et al. entitled “Thermal Diode for Energy Conversion.” The second technology uses thermotunneling of electrons through a very thin (i.e., nanometer scale) barrier. There are several patents on this technology, including U.S. Pat. No. 6,495,843 to Tavkelidze entitled “Method for Increasing Emission through a Potential Barrier.”